I someone has gained access to your router, they may have altered the login password so you will need to press the recessed Reset button to take it back to factory settings before you can get in to the settings pages.
If you want to ghange the password of the router then you need to access the internal settings of the router.
To do that you need to type in the address of the router , it will be on the router as a sticer some place or in the user manual. Examples are 192.168.1.1 and 192.168.0.1
Once you type he address you then hit enter and bring up the routers log in window and you can enter the current ser name and password. If you haven't changed it then iot will be the default username and password that came with the router.
Once in the routers internal settings you can go to the Administration section and that's where you can change the routers password.
If your wanting to change the wireless network password then you can follow the same instructions to get into the routers internal settings and then go to the wireless section and wireless security and that's where you can change the wireless network password.
Any changes that you make will have to be saved. Write down and password changes and keep them in a place where you can get them when needed.